Protecting the Whanganui river with new sewage system

Jun 2008

New Zealand

Effectively managing the sewage, wastewater
and stormwater flows in a modern
city is essential to the health and welfare
of the community and the environment.
Unfortunately in so many cities around the
world the underground networks have been
allowed to deteriorate, and now the difficulties
in upgrading them seem to be insurmountable.
In the city of Wanganui, New
Zealand, the city engineers utilised modern
plastic pipeline materials and the latest installation
technology to help them develop
their new network, and now their citizens
are benefiting from the improvements to
the environment.

The Whanganui is New Zealand’s third longest
river and originates high on the volcanic
plateau of the North Island and drains into the
Tasman Sea at the city of Wanganui, some
290 km downstream.
As with virtually every city, the Wanganui sewerage
system had developed in a piecemeal fashion
as its community had grown. Many of the pipes
were old and leaking, and there was no separation
of storm water from sewage, which at times
caused the system to overload. The old pipes
were in bad condition, and there were many untreated
industrial outflows discharging directly
into the river, significantly reducing the river’s
water quality

By the end of the 1990s it was clear that the
Wanganui sewerage system required a major
upgrade to reduce leakage and infiltration, to
separate the storm water from the sewage and
to improve the quality of the river water. The development
of a new system posed a number of
major challenges but would create a better quality
of life and a cleaner and healthier environment for
the city’s inhabitants.

In 2002 the Wanganui city council developed a
comprehensive plan to separate storm water and
sewage, and to eliminate the discharges into the
river by installing an 18 km sea outfall off nearby
South Beach. This outfall would be served by a
pumping station on the city side of the river. Allied
to this plan was the design and construction of
an interceptor main to convey screened sewage
and industrial wastewater to a new treatment
plant prior to discharge into the sea.
One of the major problems was that the new
treatment plant was on the opposite side of the
river to the existing pumping station. There was
an existing large-diameter concrete pipe under
the river, but this pipe could not withstand the
design pressure. Therefore the council was faced
with three options: build an additional pumping
station, upgrade the current submarine pipe or
build a new one.

A solution that saved NZD 2.5 million

After a feasibility study it was decided that the
most effective solution would be to insert a welded
1,000 mm diameter polyethylene pipe inside the
existing concrete pipe. The new pipe could easily
cope with the pressure, but it would need to be
600 meters long and would weigh 116 tonnes.

However, there was the added complication of
four bends in the concrete host pipe, which could
mean that the force required to insert the polyethylene
pipe would be very high. Although nothing
of this magnitude of insertion had been attempted
before in New Zealand, the city council
decided to go ahead as it was such an attractive
solution and had the added benefit that it would
save the city NZD 2.5 million compared to alternative
solutions.

Carrying 32,000 m3 wastewater daily

To resist the stresses and strains of the installation,
high-quality PE100 polyethylene material from
Borouge was selected for the manufacture of the
pipe. Specialist contractors and engineering
consultants were appointed to carry out this
unique installation.
The insertion of the pipe took place in April 2007,
and it went without a hitch. Because polyethylene
has a lower density than water, the pipe floated
inside the flooded concrete host pipe and the
pulling forces were significantly lower than had
been expected.

The complete network including the new treatment
plant was commissioned in July 2007, and today
it transports 32,000 m³ of wastewater every day,
which has lead to a great improvement in the
water quality of the mighty Whanganui River.

Greatly improved river water quality

Polyethylene provided an effective solution to an
otherwise difficult engineering problem and saved
overall cost. The challenging project was completed
without a hitch and this new pipeline forms
a key element in the city’s upgraded sewage and
wastewater network.
The new system deals much more efficiently
with the sewage, stormwater and wastewater
flows, and by eliminating discharges into the
river, it provides for a greatly improved environment
for Wanaganui’s citizens today and for
future generations.

Borouge would like to acknowledge the cooperation and
support of Darryl Mason of Tyco Flow Control, Neil Vanner
of Smythe Contractors and Colin Anderson of Inframax.